The present invention relates to a toothed belt. More particularly, the invention relates to a toothed belt having teeth having improved durability.
A toothed belt transmits power via teeth which engage with the teeth of a pulley. A high shearing stress acts on the teeth of the belt at the point where they engage with the teeth of the pulley. The fewer the number of teeth on a belt, the greater the stress, as indicated by the graph of FIG. 1. This effect deforms the teeth, resulting in wear and damage to the belt.
In order to overcome this problem, it has been the practice to increase the hardness of the teeth to thereby reduce the amount of deformation and hence improve the durability of the belt. However, because a great deal of heat is generated when a large load is repeatedly applied to the teeth as they repeatedly engage the teeth of the pulley as the belt is rotated, the polymer material of which the hardened teeth were ordinarily fabricated tended to crack. Hence, merely increasing the hardness of the belt was not sufficient by itself to overcome the above-mentioned problem.
Another approach aimed at improving the shear resistance of the teeth of a toothed belt has been to employ a multilayer-fabric structure for the teeth. This approach also suffers from certain drawbacks, which include:
(a) If two or more fabric layers are employed, PLD, which stands for pitch line difference, increases, and the wear of the teeth is accelerated due to interference between the teeth and the pulley at the time that the teeth engage with the pulley. PLD can be reduced by employing a thin fabric layer. Doing so, however, unavoidably reduces the wear resistance of the belt.
(b) If two or more fabric layers are employed, it is necessary to provide strong bonding between the fabric layers. However, it is necessary that the thickness of the rubber between the two fabric layers be small to avoid PLD problems. Moreover, the fabric layers can separate due to the heat generated by the repeated stressing of the teeth of the belt, leading to failure of the belt.
(c) Still another approach, as disclosed in Japanese Published Patent Application No. 30135/1980, is to employ two or more layers of a rigid fabric. Although this approach may result in a lowered deformation of the teeth, the amount of shear stress on the teeth is increased. Also, the possibility of separation between a fabric layer and the rubber material of the belt increases as the hardness of the fabric increases. Because, when an extraordinary large stress is applied to the belt, the tooth is largely deformed but the rigid fabric layer disposed on the rubber material is not deformed in the same way as the tooth.
With reference now to FIG. 2, which shows in cross section a segment of a toothed belt employing fabric layers, if a tooth T undergoes a large deformation, a fabric layer C will not completely follow the deformation of the tooth, and hence separation of the rubber R of the belt and the fabric layer C can occur. In order to prevent this problem, a fabric which is easily deformable and which has a large stretchability can be employed. On the other hand, employing a fabric having a large stretchability leads to crack formation at the base of the teeth T due to the large amount of deformation which the tooth will inevitably undergo.